Two-dimensional arrays of photon-counting sensing elements can be used for imaging in extremely dim (low-light) environments. Photon-counting imagers are important for many defense applications, including night vision, remote surveillance, adaptive optics and biodetection. Photon-counting imagers can also be used to determine approximately when a given photon or group of photons arrives, which is important for correlating image data, removing jitter, and compensating for predictable motion in data from or relating to micro air vehicles, satellites, and bio-fluorescence.
Photon-counting sensing elements include avalanche photodiodes (APDs), which can be biased above breakdown to operate in Geiger mode. When an APD operating in Geiger mode detects a single photon, the APD generates a pulse at a level sufficient to trigger a complementary metal oxide semiconductor (CMOS) circuit, enabling direct photon-to-digital conversion to occur in the sensing element itself. Because photon-counting imagers collect data directly in digital form, they do not suffer from readout noise or require analog-to-digital conversion, as do other solid-state sensors.
Although photon-counting imagers offer many advantages over other solid-state sensors, it can be difficult to transfer the large amounts of digital data generated by photon-counting imagers quickly enough to read out the imager. For example, suppose that the desired photon flux is one photon per ten nanoseconds, i.e., a given photon is known to arrive within a ten-nanosecond window. To read out each photon, each sensing element must be interrogated once every ten nanoseconds, which can be prohibitively fast for large numbers of sensing elements (e.g., an array with one-thousand elements requires a readout rate of 100 Gb/s). Each sensing element must transfer a bit once between successive photon detections to avoid data loss.
One approach to reducing the transfer bandwidth is to incorporate a digital counter in each sensing element. With an n-bit counter, up to 2n-1 detections can be counted by the sensing element before it must be read out. Such an architecture, while straightforward, imposes a tradeoff between the dynamic range (as limited by counter overflow) and the size of the sensing element. A counter with many bits can count a large number of detected photons, but also occupies a lot of real estate, limiting the minimum size of the pixel, and, thus, the spatial resolution of the intensity imager. In addition, every bit from the counter must be transferred during each readout to avoid loss of information, so the transfer bandwidth increases with the number of bits.
Thus, a need exists for a photon-counting imager with low transfer bandwidths, relatively small sensing elements, and high dynamic range.